With the development of hard disc drives having a high storage density, demand has been created for improvements in the performance of the spindle motors and mounting for the disc. High storage density is achieved with reduced width and spacing of the circular tracks holding the stored information. Any vibration, wobbling or deformation of the disc as it rotates will create a temporary misalignment between the read/write transducer and the circular track of stored information. Such misalignment may lead to read/write errors. Hence, high storage density depends, among other characteristics, on damping of any disc movement and the stiffness of the mounting which positions the rotating disc relative to the transducer.
Most current disc drive designs have very small vibrational damping characteristics. This is a problem because vibration can easily be induced into a mechanical system. Any mechanical system will have a series of natural frequencies, i.e. frequencies at which the system prefers to resonate. Each of these frequencies has a corresponding mode shape. In standard disc drives there are series of these natural frequencies related to the vibration of the discs. These frequencies, and their corresponding mode shapes can be predicted by mathematical modeling; the existence of these resonance modes has been verified by experimental testing. For example, FIGS. 1A and 1B are plots of the vibration measured on a spinning disc as a function of frequency with a white noise excitation. These plots will be discussed in further detail below. However, these plots clearly show both forward and backward gyro mode (see FIG. 1A), that is frequencies at which the discs vibrate back and forth like a seesaw with one rotating diameter as a stationary node. These plots also show an axial vibration of the disc (see FIG. 1B). At this frequency the discs vibrate up and down in an axisymmetric umbrella shape.
Vibration in either of these modes would obviously have the potential for causing significant misalignment between datatrack and transducer. The problem has become more significant with the use of thinner discs. Thinner discs are less stiff and result in lower resonance frequencies as well as in increased amplitude of deformation or distortion of the disc.
Some disc drive designs in the past have utilized a dampening material in the base or top cover to help improve acoustics. However, this sort of damping, while improving acoustics, does very little to stabilize the rotor dynamic system of motor and discs. Moreover, it is well known that the ball bearings used to mount most spindle motors have very low damping characteristics.
If sufficient damping could be introduced into the vibrational system of motor and discs, several problems could be improved: operational vibration, shock resistance, rotor dynamic stability problems, and parametric stability problems (e.g. stiffness dependent on angular position of rotor related to ball bearing misalignment).